Annealing steel products

ABSTRACT

CARBON DEPOSITION IS AVOIDED IN A PROCESS FOR ANNEALING STEEL PRODUCTS WITH RESIDUAL LUBRICANTS THEREON, BY INTRODUCING A LOWER ALKYL ALCOHOL INTO THE ANNEALING CHAMBER DURING THE EARLY STAGES OF THE HEATING PROCESSES. CATALYSTS SUCH AS HCI ALSO MAY BE USED.

United States Patent 3,725,141 ANNEALING STEEL PRODUCTS Uno T. Hill,Gary, Ind., assignor to Inland Steel Company, Chicago, Ill. No Drawing.Filed Nov. 10, 1971, Ser. No. 197,317 Int. Cl. C21d 1 76 U.S. Cl. 148-1615 Claims ABSTRACT OF THE DISCLOSURE Carbon deposition is avoided in aprocess for annealing steel products with residual lubricants thereon,by introducing a lower alkyl alcohol into the annealing chamber duringthe early stages of the heating processes. Catalysts such as HCl alsomay be used.

The present invention relates to a process for heat treating of steelproducts, more particularly to a process for heat treating steelproducts having residual lubricants thereon which process avoids theformation of carbon deposits on surfaces of the steel products.

During the fabrication of many types of steel products, such as steelcoil, lubricants such as oil, grease, solutions of oil or grease, oremulsions of water and oil are used to lubricate the steel during therolling, drawing, or forming operations. For instance, lubricants usedin cold rolling steel coil are generally aqueous emulsions of fattyacids, or fatty acids soaps and mineral oil. As a result of the use ofthe lubricants, the steel coils or other steel products arrive at theannealing furnace with a film of lubricant deposited thereon. While itis possible to wash the steel products with various solutions such as analkaline wash which will remove most of the lubricant, the wash step isexpensive, time consuming and not always as successful in removing thelubricant entirely.

The prior art has recognized the connection between lubricants andcarbon spots or snakey-edge formation on annealed steel coil. It isgenerally believed that when steel articles coated with such oil orgrease are subjected to an annealing treatment in a furnace, that thelubricants or grease decomposes by a mechanism which is not fullyunderstood, but the result is that carbon is deposited on the articlesbeing annealed.

Snakey-edge or carbon edge has been recognized in the prior art as aparticularly troublesome problem encountered by steel mills whichproduce cold rolled strip. According to the prior art, snakey-edge isevidenced in cold rolled steel coils by a wavy band, varying in degreesof darkness, found near the edge of strip or sheets after annealing. Thecondition is characterized by a black deposit which is sharp at theouter limit and gradually defusing into the steel background as itpenetrates towards the center of the sheet or strip. The extreme edge ofthe sheet so effected is always very clean and bright. The sharptransition from bright steel to black may occur in a distance up toapproximately 1 inch from the coil edge, while the width of the banditself can vary from a pencil line thickness up to two inches or more.The principal reason that this condition is objectionable is because iteffectively produces an unwettable surface, i.e., it is lyophobic. Thisleads to difliculties where coatings such as zinc, tin, or paint are tobe applied to the surface, and also interferes with lithograph work.

The prior art has not been consistent in describing the causes of carbonsnakes. It has been postulated that snakey-edge arises during theannealing operation from the catalytic breakdown of carbon containinggases, and that the primary catalysts for this breakdown are the finelydivided scale of iron particles arising from service breakdown duringrolling. Other prior art, such as U.S. Pat. Nos. 2,504,808, 3,167,459and 3,531,333 have attributed the snakey-edge to the breakdown orcracking of the lubricants used for rolling which are present on thesurface of the steel products being annealed. Other prior art ascribesthe problem to a combination of both theories.

Included in the prior art attempts to reduce the formation of carbonsnakes were adjustment of the quantity, type, pH, and concentration ofthe oils in the rolling oil emulsion lubricant. While some improvementwas noted by carefully adjusting these materials, the problem was noteliminated.

The prior art has also suggested that the problem of carbon depositioncan be obviated by adjusting the gas used in the annealing furnace. Theprior art has also reported that during the annealing process,variations as to the heating rate, the purging time, the furnace design,and alterations to the compositions of the gases used showed nosignificant effect on the incidents of snakeyedge. Although thesolutions to the problem suggested by the prior art appear to beoccasionally successful, they have not been uniformly successful ineliminating the deposition of carbon on the steel products during theheating operation.

According to the prior art it is known that the lubricants used on steelproducts are susceptible to cracking during the annealing processcausing the-formation of carbon as a residue which is deposited on thesteel. While it is possible under some circumstances to vaporize orvolatilize the lubricants without the deposition of carbon on the steelproducts, problems are encountered in that the vaporization temperaturesare quite high and are very nearly the same as the crackingtemperatures. Further, during conventional annealing processes, thetemperature is increased at a rate which is too rapid to permit completevaporization of the lubricants, and the cracking temperature is therebyattained before the lubricants are vaporized. The cracking of thelubricants results in deposition of the carbon on the steel products.

The present invention is based on the observation that carbon formationis obviated by the addition of methanol vapors to the annealing furnaceduring the initial heating of steel products with lubricants thereon. Itis postulated that by heating the steel with the lubricant residue to anelevated temperature, below the decomposition temperature of thelubricants, and such lubricants are reacted with alcohol vapors to formrelatively, volatile esters which may be vaporized from the surface ofthe steel products without any traces of carbon deposition. Although itis difiicult to determine precisely what reactions are taking placeduring this process, it is believed that the lower alkyl alcohols suchas methyl alcohol will react with the fatty acid components of thelubricants to form esters, and react with the fatty acids soaps, in atransesterification reaction, to form esters, which are therebyvaporized without decomposition. It is known that the vaporizationtemperature of organic esters is generally considerably lower than thevaporization temperature of the corresponding acids and of thecorresponding soap. Thus, it is believed that the process of the persentinvention converts the soaps and the fatty acids to the respectiveesters which volatilize at a lower temperature, thus, enabling thevaporization process to be completed before the temperature reaches thedecomposition or cracking temperature of the fatty acids, the fattyacids soap, or the ester per se. It is postulated that the ester form ofthe lubricants are vaporized before any significant amount of crackingtakes place, and as a result the formation of carbon on the steelsurfaces is largely obviated.

In carrying out the process of the present invention, good results havebeen obtained through the use of methanol, but the present inventioncontemplates using lower alkyl alcohols having up to about carbon atoms.How ever, methanol is preferred because of its low cost and because itforms the lowest molecular weight esters.

It has been further discovered that the reaction between the lower alkylalcohol and the lubricant components may be accelerated by the use ofcatalysts. While the preferred catalysts for use with methanol ishydrogen chloride gas, boron trifluoride has also given good results.The present invention also contemplates using other catalysts such assilica, zinc chloride, tin chloride, aluminum chloride, iron chloride,dihydroxy fiuoroboric acid, soap (such as the soaps which make up aportion of the lubricant), aluminum silicate, hydrobromic acid,hydrofluoric acid, aromatic sulfonic acid, lead, antimony, bismuth,titanium, manganese, alkali and alkaline earth compounds, alloys of tin,manganesium oxide, phosphoric acid, sulfuric acid and mixtures thereof,as well as others. It will be obvious to those skilled in the art thatmany of the aforementioned catalysts such as manganese, soaps, iron andiron chloride may be present in or on the steel product to be heattreated. It is obvious that little if any additional catalyst isrequired to carry out the reaction. However, it is contemplated that theaddition of specific catalsyts will sometimes accelerate the reaction toa desirable extent. Although it is generally preferred to use catalystswhich can be volatilized with the alcohol vapors, catalytic materialswhich are components of the steel products or the lubricants would notseem to require volatilization. Since the exact mechanism and situs ofthe reaction is not known, it is possible that a nonvolatile catalystplaced in the base of the annealing furnace, for instance would beeffective.

The amount of alcohol required to carry out the process of the presentinvention must be adequate to react with substantially all of thelubricants present on the steel being annealed. It is believed that thepresence of excess quantities of alcohol are not harmful, but arewasteful from an economic point of view. The examples herein illustratethe use of between 500 m1. and 1500 ml. of methanol in a box anneal ofabout 280 cubic feet to give a gross methanol concentration of fromabout 1.8 to about 5.4 ml. per cubic foot. However, since the annealingfurnace usually contains from about 30 to about 45 tons of steel whichweighs about 450 pounds per cubic foot, and occupies from about 130 toabout 200 cubic feet, the net free volume in the furnace runs from about80 to about 150 cubic feet, to give a net methanol concentration of fromabout 3.3 to about 19.0 ml. per cubic foot. It will be obvious to thoseskilled in the art that the alcohol concentrations may be made higher ifdesired. It is contemplated that the concentrations as low as 1.0 ml.per cubic foot may be useful.

Again the examples herein illustrate the use of or grams of HCl with 500ml. of methanol. While the process may be carried out without using anadded catalyst, it is preferred to use from about 1 to about 10% byweight of an HCl catalyst, based on the weight of the methanol.Obviously higher amounts can be used, but are not preferred because ofthe resulting expense and slight corrosion problems with HCl gas.

The following examples serve to illustrate the method of heat treatmentwithin the present invention, but it is understood that the examples areset forth merely for illustrative purposes, and many other variations ofprocess for heat treatment are within the scope of the presentinvention.

EXAMPLE 1 Three portions of a coil of cold rolled steel were deliveredto the annealing furnace approximately 2 to 3 months after rolling. Thesteel coils were relatively free from rust because of the oil filmthereon. The steel weighed approximately 10 tons per coil portion, for atotal of about 30 tons of steel. The coils were placed in a conventionalannealing furnace of the box type, which was about 6 feet in diameterand about 10 feet high, with a sand seal about the outer periphery. Theinterior volume of the furnace was about 280 cubic feet. The steeloccupied about cubic feet or about 50% of the furnace volume. Thefurnace was purged with DX gas for two hours. The heating was commenced.About the time the DX gas temperature reached during the anenaling heatcycle, one pint (about 500 ml.) of anhydrous methanol having about 15grams of dry HCl gas dissolved therein, was released into the furnace.Two hours after the first methanol was released, as the DX gastemperature approached 450 R, an additional quart (about 1000 ml.) ofmethanol was released into the furnace. The conventional annealingprocess was then continued, with the furnace temperature being raised toabout 1200 F. for approximately two hours, followed by conventionallycooling technique. The entire annealing process requires about 12 hourswhich is conventional.

The annealed coil was removed from the furnace to reveal an exceedinglybright and clean coil. It was classified as an enamel grade coil.

EXAMPLE 2 Three coils of cold rolled steel weighing a total of about 45tons were placed in a conventional annealing furnace, as described inExample 1, and sealed with sand about the outer periphery. A bottlecontaining about one pint (500 ml.) of anhydrous methanol having about15 grams of dry HCl gas dissolved therein and having a plastic capthereon was placed in the furnace. A quart can containing about 1000 ml.of methanol was covered with a plastic cover and placed in the boxanneal. The furnace was purged with DX gas for two hours. The heatingwas commenced. About the time the DX gas temperature reached 250 duringthe annealing heat cycle, the plastic caps melted, releasing themethanol and HCl. The conventional annealing process was continued, withthe furnace temperat-ure being raised to 1200 F. for approximately twohours, followed by conventionally cooling technique.

The annealed coils were removed from the furnace to reveal bright andclean coils.

EXAMPLE 3 A coil of cold rolled steel was delivered to the annealingfurnace approximately 3 months after rolling. The steel coil weighedapproximately 30 tons. The coil was placed in a conventional annealingfurnace, as described in Example 1, and sealed with sand about the outerperiphery. The furnace was purged with DX gas for two hours. The heatingwas commenced. About the time the DX gas temperature reached 150 duringthe annealing heat cycle, one pint (about 500 ml.) of anhydrous methanolhaving about 15 grams of dry boron trifluoride dissolved therein, wasreleased into the furnace. The conventional annealing process was thencontinued, with the furnace temperature being raised to 1200 F. forapproximately two hours, followed by conventional cooling technique.

The annealed coil was removed from the furnace to reveal a bright andclean coil.

In carrying out the method of the present invention it is necessary toconsider the type of lubricant and the quantity of lubricant which ispresent on the material to be heat treated. In the cold rolling of steelproducts, various lubricants are used, some of which are proprietary.

However, the lubricants are generally used either in a simple mixture ofoils and fatty acids or an 011 in water emulsion wherein the oilcomprises both mineral oil and fatty acids. Because of the mineralcontent of the water used in the emulsion, at least some of the fattyacids are generally converted into fatty acids soaps. Generally t isnecessary to determine, either empirically, or by analytical testing,the average amount of such lubricants which are present based on thearea of products being annealed. The minimum amount of methanol or otherlower alkyl alcohols employed to carry out the process of the presentinvention will be dependent, proportionally, to the amount of lubricantpresent on the products being heat treated. However, as was mentionedabove, excess amounts of methanol are not harmful.

It has been found that it is preferred to use a slight excess of theamount of methanol required, in order to produce a methanolconcentration in the furnace high enough to insure completeesterification of all lubricants. While there is no real upper limit ofthe amount of methanol used, from a technical point of view, from acommercial point of view, the cost of methanol becomes excessive as theamount of methanol is increased above the quantity required by thelubricant. The present invention also contemplates that the anneal boxshould have a free volume which equals from about 20% to about 60% ofthe volume of the steel being heat treated. It is felt that the freevolume assists the esterified lubricants in vaporizing and leaving thesteel surfaces without leaving carbon snakes.

In carrying out the process of the present invention it is generallydesirable to feed the lower alkyl alcohol to the annealing chamber afterthe purging step has been concluded, and as the heating is commenced.Preferably the alcohol is injected into the furnace when the temperaturehas reached a level which will insure volatilization of the alcohol. Itis believed that the methanol-lubricant esterification reaction onlyoccurs at elevated temperatures. It is generally desired to have thealcohol completely volatilized in order to distribute it uniformlythrough out the annealing furnace and thus into the interstices of thecoil or steel products being treated in order to react with all of thelubricant present thereon. While it is possible to add all of themethanol at one time in the annealing process, it is the preferredpractice to add it incrementally over the heating cycle, such as whenthe temperature in the furnace is between 150 and 600 F. This enablesthe methanol to react with the lubricants at a temperature below thedecomposition temperature of the lubricant, thereby permitting thelubricants to volatilize without decomposition and be carried away withthe ventilated annealing gases. As is illustrated by Example 2, themethanol may be released into the anneal box by placing it in acontainer with a thermoplastic cover, which will melt as the desiredtemperature range is achieved.

As was mentioned above, it is possible to use any of several lower alkylalcohols, such as alkyl alcohols containing up to about carbon atoms,but it is preferred to use methanol. Methanol has the lowest volatilityof the alcohols that is desired from that point of view. Further, themethyl esters which are formed by the reaction of the alcohol and thefatty acids have the lowest volatilization temperatures of the variousesters which may be formed and the methyl esters thereby providecompositions which have a more pronounced tendency to volatilize ratherthan decompose or crack. Still further, methanol is preferred since itis commercially available at nominal prices.

The atmosphere within the annealing furnace during the heat treatment isnot particularly critical with respect to the present invention. Theexamples illustrate the use of a conventional annealing gas, known asDX, which is produced by the partial combustion of methane and which hasapproximately the following composition:

Other conventional annealing furnace gases such as HN, DEOX, or HNX mayalso be used interchangeably. It is necessary that the annealing furnace'gas be substantially free from substances which tend to cause theoxidation of the lubricants or portions of the lubricants present on thesurface of the steel members being heat treated. The process of thepresent invention may also be used with atmospheres of dry, purenitrogen.

The process of the present invention may be used in any type of heatingprocess or apparatus. For instance, conventional bell chamber or boxannealing furnaces may be used, as well as other heating chambers knownto those skilled in the art.

The forms of the invention herein shown and described are to beconsidered only as illustrative. It will be apparent to those skilled inthe art that numerous modifications may be made therein withoutdeparture from the spirit of the invention or the scope of the appendedclaims.

I claim:

1. A method of heat treating steel products having lubricants thereonwhich comprises disposing the products to be heated in a heatingcharnlber, introducing a controlled atmosphere into said chamber,introducing a lower alkyl alcohol containing up to about 5 carbon atomsinto said furnace in a quantity sufficient to react with the lubricantson said steel products, heating said chamber to a temperature highenough to promote a reaction between said alcohol and said lubricantwhile ventilating said controlled atmosphere, continuing said heatingand said ventilating whereby said lubricant-alcohol reaction productsare volatilized and ventilated from said chamber.

2. A method of heat treating as described in claim 1, wherein saidalcohol is introduced in a plurality of increments.

3. A method of heat treating as described in claim 1, whereoin saidalcohol is methyl alcohol.

4. A method of heat treating as described in claim 1, wherein anesterification catalyst is introduced to said chamber.

5. A method of heat treating as described in claim 4, wherein said loweralkyl alcohol and said esterification catalyst are simultaneouslyintroduced into said chamber.

6. A method of heat treating as described in claim 4, wherein saidesterification catalyst is selected from the group consisting ofhydrogen chloride, and boron trifluoride.

7. A method of heat treating as described in claim 6, wherein saidesterification catalyst is hydrogen chloride.

8. A method of obviating carbon deposition on steel products duringannealing, which products contain residual lubricants on the surfacethereof, which process comprises surrounding said steel products with anatmosphere which includes lower alkyl alcohol vapors during the earlystages of the heating process.

9. The method of claim 8, wherein said alcohol is methanol.

10. The method of claim 8, wherein said atmosphere includes a catalystselected from the group consisting of hydrogen chloride and borontrifinoride.

11. A- method of obviating carbon deposition on steel products duringheat treatment, wherein said steel products have residual lubricants onthe surface thereof, which comprises placing said steel products in aheating chamber, and during the early stages of the heating process,surrounding said steel products with an atmosphere which 7 includes aneffective amount of a lower alkyl alcohol having up to 5 carbon atoms.

12. The method of claim 11 in which said alcohol is methanol.

13. The method of claim 11 wherein said atmosphere includes a catalyst.

14. The method of claim 13 wherein said catalyst is selected from thegroup consisting of hydrogen chloride and boron trifiuoride.

- 15. The method of claim 12, wherein the atmosphere includes from about3.3 to about 19 milliliters of methanol per cubic foot of atmosphere.

8 References Cited UNITED STATES PATENTS 2,177,031 10/1939 Tanner148-467 5 2,644,775 7/1953 Spence 14814 3,167,459 1/1965 Daguier 148-13.1

FOREIGN PATENTS 600,461 4/1948 Great Britain 148-167 10 CHARLES N.LO-VELL, Primary Examiner US. Cl. X.R.

